The present invention relates to the recycling of resin-coated steel pipes collected as waste, whether actually previously used or not, and, in particular, relates to a method for separating steel pipe from its resin coating for effective recycling of such material.
The object of the invention is to provide an effective method of easily separating steel pipe from its resin coating in such a manner that the separated resin is scraped off into chips or long strips convenient for recycling and, containing no moisture or other foreign matter, can be used in an ensuing recycling process without any preprocessing, and which is suited for large-scale industrialized processing.
Resin-coated steel pipe 4, as shown in FIG. 1, which consists of steel pipe having a thickness of 0.8 mm and an outer diameter of 26-40 mm and a coating 10 of thermo-plastic synthetic resin such as AAS acrylonitrile acryl styrene or ABS acrylonitrile butadiene styrene, having a uniform thickness of about 1 mm fixed on the steel surface with an adhesive applied there as a thin film, is rust-free, comfortable to the touch, colorful and is mechanical strong. It is widely utilized as a frame member for shelves and trolleys that are exposed to sea water in marine-related industries; as a frame member for chairs, tables, beds and stands in ordinary households; or as a frame member for benches, flowerpot stands and wisteria trellises outdoors.
The processes and equipment for manufacturing such resin-coated steel pipes are disclosed, for example, in the specifications and drawings of Japanese Patent publication S57-2498 or U.S. Pat. No. 3,941,087, and are well known.
Resin-coated steel pipes used as frame members for the various products mentioned above become, in due course, industrial waste as the products are, for some reason, no longer used or are simply discarded after losing durability. The scraps or pieces generated when pipes are cut for the construction of the various products mentioned above also become industrial waste if no use can be found for them.
The conventional method of disposing of resin-coated steel pipe waste consists of nothing better than cutting the pipes to pieces of about 15 cm in length, flattening them with a press to reduce their volume and burying them in reclaimed land, because heretofore it has been very difficult to separate the resin coating from the pipe surface due to the firm adhesion between them. This is a very easy method of disposal; however, this method precludes the recycling of otherwise useful resource, and only increases the volume of disposed waste, requiring additional land for burial.
When it has become necessary to dispose of a small quantity of waste resin-coated steel pipes, it has also been a practice to cut the pipes to a suitable length, to incinerate the resin coating in a furnace and to recycle the remaining steel pipes. This method is also easy to put into practice, but if the coating material is vinyl chloride, the incineration process produces noxious chlorine gas that causes air or other environmental pollution and badly damages the furnace walls. Further, the steel pipes that are heated undergo composition changes and produce more rust that is detrimental for the recycling process.
Another method used consists of separating the coating resin and steel pipe from waste resin-coated steel pipe pieces and recycling such materials. This method employs a super-high-pressure water jet of about 200MPa(200N/mm.sup.2) to separate the resin coating from the steel pipe. If, however, the resin pieces separated from the steel pipes and containing significant amounts of moisture are used directly for recycling, products produced therefrom are likely to develop mold. First, patterns of silver color appear on the surface of the product and badly impair the outward appearance. Second, the product does not attain a certain degree of inner strength due to the presence of moisture between the resin pieces constituting the product. The simplest and most direct method to solve this problem is to dry the resin pieces, which, however, involves a large investment for the drying equipment and a high energy cost.